Potentiometric control of bleaching of kaolin clays



United States Patent ice 3,399,960 POTENTIOMETRIC CONTROL OF BLEACHING OF KAOLIN CLAYS Robert F. Conley, Scotch Plains, and Mary K. Lloyd, Westfield, N.J., assignors to Georgia Kaolin Company, Elizabeth, N..l., a corporation of New Jersey No Drawing. Filed June 17, 1966, Ser. No. 558,286

3 Claims. (Cl. 23--110) This invention deals with the control of the bleaching ofkaolin clay-s. More specifically, it relates to the use of a special platinumelectrode for determining the most favorable contact time during the bleaching operation.

In the commercial processing and upgrading of color of natural kaolin clays, iron oxide coloring contaminants must be removed. Although a variety of chemical reagents may be employed to remove iron oxides, many of them cause a degradation of the kaolin in the operation. The most practical bleaching technique involves formulation of the clay in a water dispersion, acidification to a pH of 3.0 to'4.0, and the addition thereto of-a dithionite bleaching compound containing an active (5 0 group, or combinations of chemicals known to liberate such group, such as metallic zinc and aqueous sulfur dioxide solution, or sodium amalgam with .aqueous sodium. bisulfite solution, as well as others known in the art. By this means, the ferric ion is reduced to the ferrous ion which is readily washed out of the clay by standard cornmercial operations involving washing, dewatering, and filtering.

Since the iron oxides react only slowly with the bleaching chemical, and the rate of reaction is quite variable and unpredictable, the clay is. bleached for a sufficiently long period of time to insure as complete a reaction as possible. One of the shortcomings of dithionite reagent is that it also reacts slowly with water to produce sodium bisulfite and sodium thiosulfate as end products, neither of which is instrumental in bleaching iron oxides. Hence, bleach quantities in amounts far greater than are necessary stoichiometrically to reduce ferric ion are usually employed. It is not uncommon for quantities to be used which are as much as to 20 times in excess of the amount actually required. Commercial practice, with respect to bleaching of kaolin, involves blea'chingfor 40 to minutes at various test levels to obtain maximum brightness and whiteness of the finished kaolin pigment. In certain cases, the clay-bleach suspension is maintained many hours, even as long as a full day, before it 'is processed further in the industrial routine.

' It has been discovered that prolonged contact of the bleach with the clay may not insure a complete reaction resulting in optimum or desirable optical properties. In fact, it may result in actual degradation of the pigment quality resulting from the bleaching process. Although it is already known that degradation of kaolin can take place as a result of oxygen from the air dissolving in the clay-bleach suspension and reoxidizing the iron salts, the particular degradation referred where now is a diflerent one of a special nature. Whereas the aforesaid oxygenation mechanism generally "requires many hours or days to attain its effect, the degradation now under dismission is entirely internal, and may, in many cases, require only as little as ten minutes to manifest itself.

According to the present invention, a means is afforded whereby it is possible to fix in time a point at which the maximum amount of deoolorization is etfected in the bleaching cycle, at which point the bleaching operation is discontinued by dewaterir' g the clay-bleach suspension, washing with water, and filtering,.to. retain the maximum amount 'of whiteness in the finished dry clay pigment. This is accomplished by use of a special platinum electrode to give a measure of the electrode Patented Sept. 3, 1968 potential of the bleach-clay mixture and indicate the critical time when the bleaching operation is to be discontinued, as aforesaid.

The particular electrode system found useful for the purposes of the present invention is the saturated calomel electrode (S.C.E.), a standard reference electrode, and a special platinum metal electrode conditioned by previous prolonged contact with strong dithionite solution. For example, a platinum electrode immersed in a 1% aqueous solution of'Na S 0 for about ten hours has been found suitabl'for this purpose. When this special platinum electrode and the reference electrode are inserted in the clay-bleach mixture, the potential generated. between the two electrodes may be measured by means of a potentiometer.

"It has been discovered that kaolin clay in the bleach mixture becomes whitest in color with minimal degradation when the aforesaid special electrode system in the bleach mixture begin-s to become more negative than about 300 millivolts (where this voltage is measured by connecting the positive lead of the voltmeter to the platinum electrode) plus or minus about 25 millivolts, regardless of which operating variables are changed. The operating conditions or variables, generally, are the amount of bleach added (i.e., the bleach concentration), the temperature, the pH, the type of acidifier used, and/ or the type of bleaching agent employed.

' At its peak value, within l-Z minutes after dithionite bleach is added to a kaolin aqueous suspension, the electromotive potential, using the aforesaid special electrode system, commonly reaches values of about -500 to 600 millivolts, depending on the operating factors already enumerated. It has been observed that, regardless of the changes in operating conditions encountered, the best whiteness quality is achieved when the electromotive potential between said electrodes reaches a value of not under about 300 millivolts. If the bleaching is continued further so that the falls below this critical range, the whiteness quality decreases and also the clay is otherwise degraded in quality.

The invention will be more readily understood from the following examples which are not intended to limit the invention but are submitted to illustrated-the various phases involved therein:

Example 1 A sample of Georgia kaolin clay was formulated as a 20% solids slurry in water at 25 C., and its pH was adjusted to a value of 3.5 with a dilute solution of sulfuric acid-aluminum sulfate. This suspension was bleached with sodium dithionite at various bleach concentration levels (lb./ton of clay) for a constant period of 50 minutes in each case. Thereafter, the suspension was immediately filtered, washed with water, and dried 'at C., overnight. Table I gives the data obtained on four samples thus treated. The reflectance value, measured at 458 is measured with MgO as. l00%, all data being within plus or minus 0.2%. In this case, the higher figures denote more desirable reflectance characteristics. The whiteness index is a value difference between the reflectance at 700 and 400 -(i0.2% Here,-the lower figures denote better quality. With each sample, the contact time was continued, with agitation, until the special electrode potential reaches about 300 mv., at which point the contact time, in minutes, is recorded, and the bleaching is terminated.

TABLE I Sample Bleach Reflectance W'hiteuess Time to reach level index 300 luv.

- 3 .It will be noted that, under the conditions of Table I, as long as the bleach-clay mixture potential remained over -300 mv., increase in bleach concentration resulted in in- 4 the specified time, the slurry was processed as outlined in Example 1. The data obtained are reported in Table V.

creased color quality. TABLE V r Example 2 a Sample Bleac Reflectance Whiteness t iii tiit iit etit itii Another specimen of Georgia kaolin clay was processed temp" Index flffi i flfi similarly to that in Example 1, except that the temperature was maintained at 50 C. The results obtained are :8 33 reported in Table II. 60 87.0 13.8 24 70 87.0 1&4 1s TABLE II so 85.9 15.4 2

Sample Bleach Reflectance Whiteness Time to reach It will be noted from the foregoing that the best bright- 5 22-2 g ness and whiteness values were obtained when the clay 11 1 was in contact with the bleach for the longest time with As in the case of Example 1, the samples in Example 2 showed improvement in optical properties with increase in bleach concentration as long as the electrode potential did not fall below 300 mv.

Example 3 Another sample of Georgia kaolin was formulated as in Example 1, except that dilute sulfuric acid alone was used to bring the pH to 3.5, and the slurry was maintained at 50 C. A constant bleach level of 4 lb./ton was employed, and various bleach contact periods were used, as indicated in Table III.

TABLE III Electrode Sample Bleach time, Reflectance Whiteness Potential at min. index termination of bleach, mv.

From Table III, it is apparent that continuing substantially beyond the 20 minute period, soon after which the 300 mv. electrode potential was reached, not only did not improve further the color of the clay, but it resulted in degradation thereof. Also, it occupied plant capacity which otherwise could be used for another batch. Brightness was diminished 0.4 point, while the whiteness deteriorated 1.0 point, which values are quite significant in the clay art.

Example 4 Another specimen of kaolin, similar to that used in Example 1, was formulated as in Example 3, except that 7 lb./ton of sodium dithionite was used for bleaching. Table IV summarized the results of these tests.

TABLE IV Electrode Sample Bleach time Reflectance Whiteness Potential at index termination of bleach As was the case in Example 3, the electrode potential here indicated that the bleach reaction should have been terminated after about 20 minutes to obtain best color properties.

Example 5 A sample of Georgia kaolin similar to that in Example 1 was formulated as a 20% solids slurry, and the pH was adjusted to 3.0 with sulfuric acid. The acidified slurry was bleached with 3 lb./ton for 40 minutes in each case, with temperature varied, as indicated. After bleaching for the electrode potential being more negative than 300 Example 6 A specimen of Georgia kaolin was formulated as in Example 1. The pH was adjusted to 3.5 ,with a dilute sulfuric acid-aluminum sulfate solution, and the slurry was bleached for 40 minutes at 50 C. with commercial zinc dithionite at various bleach concentration levels. Table VI summarizes the results obtained.

Although zinc dithionite is more effective for bleaching the clay, in that it achieves the desirable optical properties in a shorter bleach period than the corresponding sodium salt, the data in Table VI demonstrate that the period during which the potential in the bleach mixture is more negative than 300 mv. results in the best optical properties.

From the foregoing we can summarize that the bleach mixture electrode potential should not be permitted to drop below a negative value of 300 mv., regardless of the changes in the operating conditions of the mixture, and the longer the time of contact of the clay with the bleach at an electrode potential more negative than 300 mv., the better the optical properties of the clay.

The lack of a whiteness index figure for Sample U was due to the fact that the instrument pen ran out of ink. The parenthetical time period given for Sample AC is due to the fact that it is an estimated figure between 8 and 10.

We claim:

1. A method of improving the optical properties of kaolin clay, comprising:

mixing said clay with an aqueous dithionite bleaching solution to form a slurry,

inserting in said mixture a saturated calomel cell electrode and a dithionite-conditioned platinum electrode in a manner so as to enable reading the electric potential between said electrodes in said bleaching mixture,

maintaining said bleach mixture, with stirring, while the negative electric potential between the electrodes is becoming more positive and discontinuing the bleaching operation before the said potential drops below a negative value of about 300 millivolts. I p

2. A method of improving the optical properties of kaolin clay, according to claim 1, in which the bleaching solution comprises a sodium dithionite.

3. A method of improving the optical properties of kaolin clay, according to claim 1, in which the :bleaching solution comprises a zinc dithionite.

References Cited UNITED STATES PATENTS 2,601,203 6/1952 Bennett et a1. 2041 X 2,758,010 8/1956 Rowland 23-110 2,773,019 12/1956 Waddell 204-1 3,290,161 12/1966 Sheldon et a1 23-110 X OSCAR R. VERTIZ, Primary Examiner.

A. GREIF, Assistant Examiner. 

1. A METHOD OF IMPROVING THE OPTICAL PROPERTIES OF KAOLIN CLAY, COMPRISING: MIXING SAID CLAY WITH AN AQUEOUS DITHIONITE BLEACHING SOLUTION TO FORM A SLURRY, INSERTING IN SAID MIXTURE A SATURATED CALOMEL CELL ELECTRODE AND A DITHIONITE-CONDITIONED PLATINUM ELECTRODE IN A MANNER SO AS TO ENABLE READING THE ELECTRIC POTENTIAL BETWEEN SAID ELECTRODES IN SAID BLEACHING MIXTURE, MAINTAINING SAID BLEACH MIXTURE, WITH STIRRING, WHILE THE NEGATIVE ELECTRIC POTENTIAL BETWEEN THE ELECTRODES IS BECOMING MORE POSITIVE AND DISCONTINUING THE BLEACHING OPERATION BEFORE THE SAID POTENTIAL DROPS BELOW A NEGATIVE VALUE OF ABOUT -300 MILLIVOLTS. 